BUILD: compiler: workaround a glibc madness around __attribute__()

BUILD: compiler: workaround a glibc madness around __attribute__()

For whatever reason, glibc decided that the __attribute__ keyword is
the exclusive property of gcc, and redefines it to an empty macro on
other compilers. Some non-gcc compilers also support it (possibly
partially), tinycc is one of them. By doing this, glibc silently
broke all constructors, resulting in code that arrives in main() with
uninitialized variables.

The solution we use here consists in undefining the macro on non-gcc
compilers, and redefining it to itself in order to cause a conflict in
the event the redefinition would happen afterwards. This visibly solved
the problem.